The prior-art directional sensor is described in a textbook by E. F. Sawaransky and D. D. Kirnos: Elemente der Seismologie und Seismometrie (Elements of Seismology and Seismetry), pp. 339-344, published in 1960, hereinafter referred to as "Sawarensky and Kirnos" for short.
Directional sensors for solid-borne sound are especially needed in the low-frequency range below ca. 1000 Hz and in the infrasound range below ca. 25 Hz for locating artificial and natural seismic sources which are sites of any king of underground and above-ground extracting blastings, including the seismic prospecting of deposits and layer wave seismics, the seismics of pile drivers, machines and vehicles, earthquakes, rock bursts, gas explosions, and knocking signs of spills, as well as ground vibrations induced by sound transmitted by air.
The particularly important parameters for localization of a seismic focus are the azimuth alpha (a) and the emergence of surfacing angle epsilon (a) of the seismic ray at the site of observation.
Varying from the above-described state of the art of this class (Sawarensky and Kirnos), so-called arrays are currently preferably used in earthquake seismics (cf. H. P. Harjes and M. Henger; Array Seismology, published in Zeitschrift fur Geophysik, 1973, vol. 39, No. 6, pp. 865 ff., hereinafter referred to as "harjes and Henger" for short. Array seismology uses compound systems of seismometers arranged over an area, which are arranged in various configurations depending on their task, e.g., for locating near and remote earthquakes or for detecting or verifying nuclear explosions. Such configurations form can be an L-shaped, cross-shaped, triangular, or, more recently, even a circular arrangement see (publication of the Royal Norwegian Ministry of Foreign Affairs, NORSAR Norwegian Seismic Array "Seismological Verification of a Comprehensive Nuclear Test Ban", pp. 4-11).
Such arrays are expensive in every respect, especially because their extent may reach a few kilometers (small arrays) or several hundred kilometers (large arrays).
In array seismology, the determination of the azimuth of the seismic ray is based essentially on the measurement of the different arrival times of the wave at the individual seismometers. This procedure is called the "position correlation method" (cf. elsewhere in Harjes and Henger, pp. 880 ff., especially in connection with FIG. 7). Using digital measurements and filtration methods and special evaluation methods (beam forming), the angular accuracy of the azimuth alpha reaches .+-.5.degree. and at times up to .+-.3.degree..
The approach suggested by Sawarensky and Kirnos with the prior-art directional sensor of this class, which was described in the introduction, differs considerably from array seismology and the extremely expensive apparatus associated with it.
The prior-art directional sensor uses the azimuthal correlation. This measurement method consists of observing the phase and the amplitude of the seismic waves as a function of the azimuth alpha of the vibration direction of a plurality of horizontal seismometers at a measuring point. In contrast to the expensive array seismology, these horizontal seismometers are concentrated into a space with a transverse extent of a few decimeters. Sawarensky and Kirnos performed their seismologic recordings with 6 to 7 horizontal seismometers (cf. Sawarensky and Kirnos, p. 341, FIG. 15). The amplitude A of the measuring vibration signal generated by a geophone on the chart recording during the transition from one channel to another now changes according to a cosine law: EQU A=A.sub.max cos (beta-alpha).
Here, beta is the azimuth of the direction of vibration of the seismometer for the channel in question, and alpha is the azimuth of the ground vibrations. Consequently, the curve that describes the dependence of the sensitivity of the channel on its azimuth beta, the so-called directional diagram of the channel, has the shape of a cosine line curve or, --represented in polar coordinates, the shape of a circle or double circle. Now, ##EQU1##
For the longitudinal waves P, the recorded amplitudes will be highest for the devices (geophones) in which he azimuths of their axes coincide with the azimuth of the seismic ray (alpha).
The directional diagram of a simple azimuthal arrangement and the corresponding recording of simple waves (e.g., of P) are shown by Sawarensky and Kirnos on p. 341, FIG. 16.
The azimuth alpha of the seismic ray can thus easily be determined from the recording of the longitudinal P waves. Alpha corresponds to the azimuth beta of the channel, whose amplitude has the maximum, or it is at right angles to the beta value (i.e., beta=pi=/2+alpha) for which A=0. This ambiguity is eliminated by using a vertical seismograph. Using such an arrangement alpha can be determined somewhat more accurately than by means of the usual three-component arrangement because a greater number of projections is used.
Sawarensky and Kirnos saw a certain improvement of the determination of the azimuth in azimuthal arrangements with inclined seismometers which are set up such that no phase shift by pi, occurs in the recordings of all measuring channels, and the amplitude alone are sufficient for a reliable correlation (cf. Sawarensky and Kirnos, pp. 342, 343, FIGS. 17-19). The apparent emergence angle epsilon is now obtained from the ratio of the maximum amplitude A.sub.max to the minimum amplitude A.sub.min in the directional diagram. Now, ##EQU2## applies (cf. Sawarensky and Kirnos, p. 343, in conjunction with p. 342, FIG. 18) when psi, i.e., the slope angle of the apparatus against the horizontal, satisfies the condition psi&gt;pi/2-epsilon.
The use of the directional sensor of this class, which is described in the introduction (Sawarensky and Kirnos), was limited as a consequence of the consumption of a large amount of photographic recording paper and the frequent replacement necessary in the case of stationary observations.
The prior-art directional sensor (Sawarensky and Kirnos) is highly attractive compared with the above-mentioned seismological arrays due to its compact size, which permits measuring recording of one point along with high mobility and a relatively low cost of installation. However, the prior-art directional sensor (Sawarensky and Kirnos) was found to be in need of improvement in terms of its handling and the related reduction of the accuracy of reading. An even simpler design, resulting from improved handling, would also be desirable.